A biological study of Durvillaea antarctica (Chamisso) Hariot and D ...

89
would receive little sunlight, Their photosynthetic rate would be
small compared ... lith plants growing ;i:n places where the tidal range
is less than 3 m.
The D. anta~atiDa fronds could only float on the surface if
the kelp band grew higher up the shore at about 1-2 m below EHWS.
At that level they \lIould be emergent for 9-10 hours when sea conditions
were calm. In that time, large D. antapctica can lose 40% of their
t'/eight, and SIllalle:r:' specimens become completely shrivelled. It is
unlikely I therefo-,::-e,that D. antaY'ctiea could survive in the upper
littoral on the coasts of Pa,tagonia and east Tierra del Fuego.
(d)
The absence of Durvillaea on south African coasts:
stevenson (1939/ 1944, 1948) divided the South African
littoral into three floristic and faunistic areas: subtropical (the
Natal coast) f warm temperate (south coast from Cape Town to the Natal
Border) and cold 'tempe~ate (north from Cape Town into South West
Africa) .
Maximum sea temperatures in the southern part of the subtropical
region are comparable with northern New Zealand and southern New
South Wales where DurviZlaea grows. Winter temperatures are
considerably warmer (Table 4.2) I and it is possible that. this ma.y be
the precluding factor. Another possible factor is that currents in
this region (t40zambique and Alguas Currents) originate in the tropics
of the Indian Ocean where there is no Du'('villaea. As Stevenson
pointed out, tl~ littoral organisms are correspondingly of Indowest-Pacific
origin.
The absence of DurvilZaea from the cold temperate and ,,,arm
temperate coasts is more difficult to explain. According to
Discovepy reports, the cold inshore waters off th(, west coast
originate from an upwelling of subantarctic wo.ter knONn as }'mtarctic
Intermediate Water. This flows northwards along the coast as the
cold Bengue la CUrrent" Steven son et dL (19 36>.record:d 1 i t tle difference
in seaSl.lrface ·teropera'cure between Fa.lse Bay, neax Cape 'I'm'In, and
~Ialvis Bay 1100 kin further north at 23"S. Sea. surface t.emperatures
a.long th is cold tel.1'lpe.ra te coast are thus comparable 1.vi th centr.al
ChUe and central and southern New zealand, and are only sl:i.ghtly
warmer 'than at Gough 1., the closest population of Durvillaea. In
the warm te:rnperate region temperat.ures are higher, hut they are still
comparable with temperatures in northern Ne,,, Zealand ox' on the::
Victor ian coas t where IJw:>vi llaea g:COlil!'l.